The purpose of this study is to 1) evaluate ZDV+3TC vs. d4T+ritonavir vs. ZDV+3TC+ritonavir with respect to the change in plasma HIV-1 RNA copy number from baseline to 48 weeks in stable HIV-infected children with greater or equal to 16 weeks of prior continuous antiretroviral therapy, and 2) evaluate the safety and tolerance of ZDV+3TC vs. d4T+ritonavir vs. ZDV+3TC ritonavir based upon laboratory and clinical toxicities.